The use of varactors, which are actually semiconductor diodes having measurable and predictable capacitance, as frequency multipliers has been quite usual and many are sold every year for this purpose. The usual arrangement is that an alternating voltage is supplied across the varactor which is connected into the circuit such that it becomes self-biased to operate in a "snap" condition. As is well known to those skilled in the art, in this condition the varactor generates numerous harmonic frequencies, and through the use of an appropriate band pass filter, a desired harmonic frequency may be selected. It is quite common to use a varactor in this manner to generate frequencies of two, three or four times the input frequency. Other multiples are also possible, sometimes at the cost of some complication in associated circuitry. It is inherent in the operation of such circuits that the harmonic frequencies tend to build up and decay somewhat slowly, thus giving rise to envelopes with relatively poorly defined leading and trailing edges.
Where operating at high frequencies (one to five GHz) and at comparatively high power levels (100 watts), it is often difficult to switch the multiplied frequencies to create sharp cut-off and/or turn-on of the signal without adding additional components and related high insertion loss. In the case of echo ranging devices such as radar altimeters, it is essential that the leading or trailing edges of such multiplied pulses be precisely defined and controllable. The usual method of accomplishing this switching function is to place a gating circuit downstream of the varactor which operates to switch the varactor output off and on as desired. This gate circuit becomes an element of cost and, in some applications, creates a special problem in that it also adds an increasing insertion loss as the developed output power increases. In some applications this insertion loss is either intolerable or, at the least, very undesirable. There is, therefore, a need for a varactor frequency multiplying circuit which can be switched controllably and precisely and without incurring additional insertion losses as a trade-off for increasing output power.